This application is based on application No. 98-7854 filed in the Korean Industrial Property Office on Mar. 10, 1998, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to and an active negative material for lithium secondary battery, a method of preparing the same and a lithium secondary battery using the same, and more particularly, to an active negative material useful for fabricating a lithium secondary battery having a high capacity.
(b) Description of the Related Art
In recent years, the development of miniaturized portable electronics provokes needs for a secondary battery having a high capacity as well as a light weight. From the viewpoint of the capacity improvement per unit weight, the lithium secondary battery is preferably adopted because lithium has a high standard potential as well as a low electrochemical equivalent weight.
FIG. 1 is a schematic cross sectional view showing the general structure of the coin-typed lithium secondary battery. As shown in FIG. 1, the lithium secondary battery includes a positive electrode plate 40 having a collector 1 made of nickel and an active material layer 10 coated on the collector 1, a negative electrode plate 45 having a collector 1xe2x80x2 made of copper and an active material layer 30 coated on the collector 1xe2x80x2, and a separator 25 interposed between the positive and negative electrode plate 40 and 45. The positive and negative electrode plates 40 and 45 essentially form an electrode plate assembly together with the separator 25. The electrode plate assembly is inserted into an opening portion of a battery case 5 internally surrounded with a gasket 20 while receiving an electrolyte 15 therein. The opening portion of the battery case 5 is covered by a cap 35.
FIG. 2 is a schematic cross sectional view showing the general structure of the cylindrical-typed lithium secondary battery. As shown in FIG. 2, the lithium secondary battery includes positive electrodes 50, negative electrodes 55 stacked in order and separated by a separator 60. The layered structure is wound a number of times in a spiral form to obtain an electrode winding body. The electrode winding body is inserted into a battery can 90 including nickel plated steel insulating plates 80, 85 respectively inserted into the top and bottom of the battery can 90. For the purpose of collecting electricity of the negative electrode, one end of a nickel negative lead 75 is pressure-connected to the negative electrode, and the other end thereof is welded to the battery can 90. In addition, in order to collect electricity of the positive electrode, one end of an aluminum positive electrode lead 70 is attached to positive electrode 50, and the other end is welded to battery cover 100 having current interrupting mechanism 105.
Lithium-containing transitional metal oxide such as LiCoO2, LiNiO2, LiMn2O4 and LiNixCo1-xOy are preferably selected for the positive electrode active materials in conjuction with a polyethylene-based porous film for the separator.
As for the negative electrode active materials, lithium metal is attractive because it has a light weight and high capacity per unit weight to thereby output high voltage in the battery use. However, the use of lithum metal for the negative electrode material reveals serious defects in a cycle life and stability of the battery because the lithium metal is highly reactive with the electrolytic solvent and easily forms needle dendrites during cyciling, casusing destruction of the separator and a short circuit. In order to avoid the defects, lithium alloys are employed as the negative electrode active material instead of the lithium metal but yet reveals similar problems.
Alternatively, carbon materials, which can reversibly accept and donate significant amounts of lithium without affecting their mechanical and electrical properties, are proposed for the negative electrode active material.
The carbon materials adapted for use in a battery are generally spherical type carbon materials and fiber type carbon materials. A method for preparing the spherical type carbon material is disclosed in Japanese Patent No. Hei 1-27968. In the method, a coal tar pitch is heat-treated and centrifuged to thereby produce small spherical particles having an optical anisotropy (referred to hereinafter more simply as mesophase particles). Alternatively, a coar tar pitch is centrifuged to obtain supernatant and the supernatant is heat-treated, obtaining mesocarbon microbeads.
However, the aforementioned techniques are not economical and the manufacturing process is complicate because the centrifugation step should performed for obtaining mesophase particles. Furthermore, mesophase particles is only used for preparing spherical carbon materials, the total yield is low.
It is an object of the present invention to provide a negative active material useful for fabricating a lithium secondary battery having a high capacity.
It is another object of the present invention to provide a method of preparing the negative active material with simple manufacturing process and high yield.
It is another object of the present invention to provide a lithium secondary battery using the negative active material.
In order to achieve this object and others, the present invention provides a negative active material including a graphite-like carbon material having an intensity ratio I(110)/I(002) of an X-ray diffraction peak intensity I(002) at a (002) plane to an X-ray diffraction peak intensity I(110) at a (110) plane of less than 0.2. The negative active material is prepared by dissolving a coal tar pitch or a petroleum pitch in an organic solvent to remove insoluble components therefrom, heat-treating the pitch at a temperature in the range of 400 to 450xc2x0 C. for four hours or more to thereby produce at least 50 weight percent of mesophase particles based on the pitch, coking the pitch including the mesophase particles, carbonizing the coked pitch, pulverizing the carbonized pitch and graphitizing the pulverized pitch.
The lithium secondary battery includes a negative electrode having the negative active material, a positive electrode having a lithium containing material that can reversibly intercalate and de-intercalate lithium ion and a non-aqueous electrolyte.